You could make two forms with 2 different actions
<form action="login.php" method="post">
<input type="text" name="user">
<input type="password" name="password">
<input type="submit" value="Login">
</form>
<br />
<form action="register.php" method="post">
<input type="text" name="user">
<input type="password" name="password">
<input type="submit" value="Register">
</form>
Or do this
<form action="doStuff.php" method="post">
<input type="text" name="user">
<input type="password" name="password">
<input type="hidden" name="action" value="login">
<input type="submit" value="Login">
</form>
<br />
<form action="doStuff.php" method="post">
<input type="text" name="user">
<input type="password" name="password">
<input type="hidden" name="action" value="register">
<input type="submit" value="Register">
</form>
Then you PHP file would work as a switch($_POST[‘action’]) … furthermore, they can’t click on both links at the same time or make a simultaneous request, each submit is a separate request.
Your PHP would then go on with the switch logic or have different php files doing a login procedure then a registration procedure